1. Field of the Invention
The present invention relates to a method for qualifying the cleanliness of the cylinder valve on a gas cylinder, which includes an optional cleaning of the cylinder valve if necessary. The main standard for qualifying cylinder valve cleanliness is based on the number of particles adhering to the interior wall of the flow path.
2. Description of the Related Art
Processes for fabricating semiconductor devices and electronic equipment require that the distribution system for the process gases used, for example, the reactive gases and carrier gases, be as clean as possible. The sources of contamination that can impair the cleanliness of such gas distribution systems are believed to be the gas itself, as acquired, the gas cylinders filled with and holding the gas, and atmospheric air that has become admixed during the filling process.
Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 7-134090 (134,090/1995) discloses a method for qualifying (evaluating) and processing the cylinder body in view of the contamination by metal microparticles (particles) from the interior wall surface of the cylinder body of gas cylinders. In this method, the cleanliness of the cylinder body is evaluated by discharging the interior gas while applying ultrasound to the cylinder body and by analyzing the discharged gas.
However, the cylinder valve of a gas cylinder is considered to be a more significant source of gas distribution system contamination than either the gas as supplied or the cylinder body, because the cylinder valve is subjected to processes that result in the production of larger numbers of particles. For example, friction between the valve body and valve seat, corrosion of the components within the cylinder valve by corrosive gases, and reactions between air and highly reactive gases can all lead to contamination. Moreover, taking as an example the widely used 47-liter cylinder, the linear gas velocity within the cylinder valve can be 1,000 times or more than that within the cylinder when gas is supplied from the cylinder at a prescribed flowrate, which causes the gas flow-induced particle desorption probability to be much larger within the cylinder valve than within the cylinder.
In U.S. Pat. No. 5,676,762 there is disclosed a method for cleaning ultrahigh purity gas distribution systems (which include the cylinder valves of gas cylinders). This method cleans all the components within the flow path of a gas distribution network using a solvent as a cleaning fluid. This method is very useful for clearing the distribution systems for corrosive gases such as HBr, HCl, NH.sub.3, and the like.
The deposition of large amounts of particles on the interior surfaces of the cylinder valves used with highly reactive gases such as SiH.sub.4, PH.sub.4, B.sub.2 H.sub.6, and so forth has been observed. These particles are composed of the reaction products, such as SiO.sub.2 and P.sub.4 O.sub.10, resulting from the reaction of these highly reactive gases with air. These particles adhering on the interior surfaces of the cylinder valve debond during gas filling and gas removal and enter the gas flow, and in this manner contaminate the incoming or outgoing gas. Due to its use of a solvent as a cleaning fluid, the method of U.S. Pat. No. 5,676,762 is disadvantageous from a safety standpoint for the cleaning of cylinder valves which are used with highly reactive gases such as SiH.sub.4, PH.sub.4, B.sub.2 H.sub.6, and so forth.
An object of the present invention therefore, is to provide a method that, by virtue of being different in its aspects from the prior art, is a very safe method for evaluating and qualifying the cleanliness of cylinder valves and in particular is optimal for application to the cylinder valves of gas cylinders for highly reactive gases.
An additional object of the present invention is to provide a qualification method in which, based on the results of the evaluation of the cleanliness of the cylinder valve, the cylinder valve can be cleaned subsequent thereto.
These and other objects of the present invention will become apparent upon a review of the following specification, the Figures of the Drawing, and the claims appended hereto.